Income Taxes - Components of Net Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Net deferred income tax assets (liability):
Depreciation and amortization	$ 48	$ 238
Accrued expenses and reserves	47	148
Deferred income tax assets, Deferred revenue		508
Deferred income tax (liability), Deferred revenue	(63)
Net operating loss carryforwards	17,443	14,561
Research and development credit carryforwards	3,337	3,037
Stock-based compensation	1,006	940
Other	19	10
Gross deferred tax assets	21,837	19,442
Less: valuation allowance	(21,830)	(19,442)
Deferred income tax assets, Deferred revenue		$ (508)
Net deferred tax assets	$ 7